In order to measure a wide range of dose rates which range from a natural radiation level at ordinary times to a high radiation level at the time of an accident, two dose rate monitoring devices (or a radiation monitoring apparatus) are installed in and around a nuclear reactor facility, a spent fuel reprocessing plant and the like (for example, Patent Documents 1-6). Each of the dose rate monitoring devices is equipped with a radiation detector of a different kind. Contrary to the intention to share the measurement range for measuring dose rates at the same measurement point, arranging detecting devices side by side enlarges the size of a whole apparatus, and moreover, poses an obstacle to the incidence of space radioactive rays, and makes an impact on the accuracy of the measurement.
It is eagerly anticipated to provide a dose rate monitoring device which can be applied to a wide range of dose rates in a single body, also from the viewpoint of cost reduction and space saving. As a measure for solving this subject, proposed is a radiation monitoring apparatus (or a dose rate monitoring device) which includes a single detection part on which a low range detector and a high range detector are arranged. The low range detector employs a scintillation detector and the high range detector employs a semiconductor detector. The radiation monitoring apparatus switches the range in measurement depending on the magnitude of a dose rate and outputs the dose rate.
When switching between a low range and a high range is performed for outputting a dose rate, a difference in level will be produced in the measured values, due to the difference in energy characteristics of the radiation detectors. In order to reduce the difference in level, the radiation monitoring apparatus according to Patent Document 1 measures a wave height spectrum in the output pulse from at least one of the detectors, and estimates the energy of an incident radioactive ray. The radiation monitoring apparatus is configured to cancel a big difference in level which is produced at a switching point of the measurement ranges, by uniting this estimated value with one of the energy characteristics with respect to the upper and lower dose rate domains which include the switching point.
In the radiation monitoring apparatus according to Patent Document 2, three scintillation detectors are arranged; with one detector on the head face of the cylindrical scintillator of a scintillation detector and the other two detectors on the side faces of the cylindrical scintillator, each detector with a 180 degree separation. By making efforts in the arrangement of radiation detectors, the radiation monitoring apparatus is configured to reduce differences in level which are to be produced at the time of range switching due to the directional dependencies (differences in sensitivity due to the incident direction of a radioactive ray) of a low range detector and a high range detector. In both Patent Documents, a NaI (Tl) scintillation detector, for example, which employs a thallium activated sodium iodide scintillator, is used as a scintillation detector.